1. Field of the Invention
The present invention pertains to an electrical excitation device for the coil or coils of an electromagnetic actuator, and especially of an electromagnetic actuator with linear movement.
2. Description of the Prior Art
The coils of electromagnetic actuators with linear movement have a number of important applications in a very wide variety of control actuators, such as electric door openers, electromagnets with mobile cores, loudspeaker motors, linear motors for servo-controlled mechanisms, control devices for the tuning piston of a microwave resonant cavity etc.
There is a known way to achieve the electrical power excitation of the coil of an electromagnetic linear motor by means of a bridge of power transistors, the control electrodes (gates in the case of field-effect transistors and bases in the case of bipolar transistors) of which are driven by means of a pulse-width modulation and chopping amplifier. When the reference voltage is zero, these control electrodes are driven by symmetrically chopped signals (square-wave signals for example) so that the mean current in the coil is zero. When this reference voltage has a value, for example a positive value, tending to make the core of the actuator move in a first direction, these signals are chopped dissymmetrically in a first direction by the modulator, so that the mean current in the coil has a certain value, for example a positive value, that depends on the degree of the modulation, i.e., in fact on the amplitude of the error voltage that controls the modulator. By contrast, when this same reference voltage has a value, negative this time, tending to make this core move in the other direction, these very same chopped signals are chopped dissymmetrically in the other direction. This gives a negative value to the above-mentioned mean current flowing through the electromagnetic actuator.
One drawback of this conventional device is the risk of inducing spurious voltages due to this chopping of power at varying levels of frequency. As a rule, the movable magnetic core is associated with a position sensor which operates in the general position feedback loop for this core, with its associated electronic processing system being often placed in the same casing as the above-mentioned power amplifier. This raises problems in controlling spurious voltages that are all the more severe as the signal given by this position sensor must be as precise as possible.
Furthermore, especially in the case of linear motors with short response times, it is desirable to use coils with a small number of turns having a high current going through them, rather than coils with a large number of turns having a lower current going through them, so as to obtain optimum heat dissipation. It follows that the self-inductance of the linear motor then has a low value so that, when the reference voltage is zero and when, consequently, the successive voltage pulses at output of the motor are balanced in both directions, although the motor is mechanically fixed, its coils are crossed by an AC current with an rms value that is far from negligible given the excessively small damping due to the excessively low self-inductance of these coils. A non-negligible electrical power is thus dissipated at rest in these coils.
To obtain a low rms current at rest in these coils, it would be necessary to work with a very high chopping frequency (of the order of one to several megahertz). This, firstly, is incompatible with efficient working of the power transistors and, secondly, does not resolve the above-mentioned problem of controlling spurious voltages.